The 270 is already overbore. What would you consider to be an improvement? Re-barreling to 280 would be an improvement because of the 9:1 twist. That reduces the effects of overbore by allowing heavier bullets which can improve long-range trajectory; however, I don't know if that is what you're looking for.

Go to the local Hobby Shop and buy a piece of brass tube. Take a 270 MT cartridge to select the size of tube which will will fit over the case neck.

This will be your "drop tube". The load the 270 Win, starting without using the drop tube getting as much of IMR 7828ssc as you think is a good starting load for the 140 grain bullet you will use.

Keep increasing powder, using the drop tube, until you get decent pressure and check the velocity. You may be surprised, in a pleasant way, with the potential of that powder in the winny. AGAIN, be careful........